1. Field of the Invention
The present invention relates to steel for a gear that is the best suited for molding cogs by component rolling, a gear that is made of the steel for the gear and is a drive plate gear used in starting of an engine, and a method for producing the gear.
2. Description of the Related Art
As steel for a gear of this type, there is S35C material defined in the Japanese Industrial Standard (JIS),for example. The S35C material includes C: 0.32 to 0.38%, Si: 0.15 to 0.35, Mn: 0.60 to 0.90%, P: 0.03% or less, S: 0.035% or less, and the balance of Fe and unavoidable impurity, by weight.
The drive plate gear transmits rotation of an engine to a transmission side and transmits rotation of a starter motor to the engine by a gear provided to an outer periphery of the drive plate gear.
Such a drive plate gear is usually produced by cold working by using the above S35C material. In other words, in producing the drive plate gear, a thick portion is molded on an outer periphery portion of the S35C material provided in a shape of a disc, and then, the gear is molded on an outer periphery of the thick portion by component rolling.
However, because hardness increases in molding the thick portion when the above S35C material is used, end portions of a cog (end portions in a width direction of the cog) are likely to crack in molding the gear. There is also a disadvantage of extremely short life of a gear-molding tool because of high hardness of the thick portion.